Toss the blackberries, caster sugar and 1 tbsp of the lemon juice in a small pan. Heat gently for 3-4 mins or until the berries start to burst. Leave to cool.
Meanwhile, mix together the crushed biscuits and melted butter. Spoon into 2 glasses. Gently fold the rest of the lemon juice, the lemon zest and icing sugar into the mascarpone or ricotta. Divide the lemony cheese between the glasses, then spoon over the blackberries and their syrupy juices.